Illuminator and production method

ABSTRACT

An illuminator ( 1 ) has an array of cavities ( 3 ) are drilled in a substrate having an FR4 electrically insulating body ( 7 ) plated with copper conductors ( 6 ( a ),  6 ( b )). Further patterning of the substrate provided electrical conductors ( 3 ( b )) in the cavities to act as both part of the light drive circuit and reflectors in the cavities. The drilling is performed to the full depth of the FR4 body ( 7 ) until underneath plating ( 6 ( b )) is reached. A further Cu plating ( 4 ( b )) is applied on the platings ( 6 ( b )) so that they together form a heat spreader under each cavity ( 3 ). A layer ( 8 ) of resin incorporating thermally conductive particles is bonded to the underside surfaces, both of the FR4 ( 7 ) and of the platings ( 4 ( b )). A heat sink ( 9 ) is bonded to the layer ( 8 ). Good optical efficiency and thermal dissipation is achieved, despite the fact that conventional PCT processing techniques are used.

INTRODUCTION

1. Field of the Invention

The invention relates to an illuminator and to a method for producing anilluminator.

2. Prior Art Discussion

Light emitting and infrared emitting diodes (referred to hereinafter as“LEDs” are widely used as indicators and as sources of illumination fora wide variety of applications. In order to ensure maximum efficiency,reliable operation, and a long lifetime it is necessary to take variousmeasures in assembling the LEDs into packages or into housings such asare typically used in illuminators.

For the use of single or small numbers of LED dies, the through-pinpackage (“T-Pack”) has been developed. In this package the die sits in ametal reflective cavity which enables good optical efficiency byreflecting light from the sides of the chip towards a moulded lensintrinsic to the package. The thermal resistance of the package isacceptable for some applications, but limited by the length of the pinsconnecting it to the rest of the assembly. This T-pack, andsurface-mount variations of it, has become commonplace for manyapplications.

There are also in existence techniques for manufacturing light arrays byfor example using metallized plastic cavities, incorporating opticalreflectors, such as are used for scanning sources in photocopyingmachines. These typically work well, but are not capable of achievinghigh LED densities.

For applications such as medical, machine vision, and signage, it isoften desirable to try to locate a dense 1 or 2 dimensional array ofLEDs close together. However, the physical dimensions of T-Packs orsurface mount packages limit the densities which can be achieved.

An alternative approach is to work with bare dies, and mount themdirectly on a circuit substrate such as FR4, making electricalconnections from the circuit to the back of the chips with conductivedie-attach epoxy and/or to the front of the chips with wire bondingtechniques. In this case a high density (for example. 4 dies per sq.mm.for 0.3 mm square dies) of LEDs and correspondingly high brightness canbe achieved. If the chips are being operated at high current levels itmay be necessary to reduce the thermal resistance of the assembly byusing a thin ceramic substrate instead of FR4. However, in both of thesecases, although there is an improvement in radiant energy densitycompared with the typically achievable densities with T-Pack or surfacemount packages, the improvement is partially offset by an opticalefficiency reduction due to loss of light emitted from the sides of theLED dies, which is not collected by lenses used in these assemblies.Also, the heat which is generated by the density of the LEDs can causereliability problems, can reduce useful life, and can reduce opticalefficiency.

JP62235787 describes an arrangement in which a metal substrate of goodthermal conductivity has recesses, each containing a light source. Toavoid electrical shorting problems and to provide circuit connectivityinsulator layers and conductor elements are separately deposited bothabove and in the recesses. It appears that this arrangement leads tocomplexity for electrical circuit manufacture arising from the fact thatthe substrate is of metal.

This invention is therefore directed towards providing an illuminatorand production method which enables a good density of LED sources to beachieved in an optically efficient manner, combined with low thermalresistance. Another object is to achieve this by using techniques whichare compatible with common printed circuit board (PCB) manufacturingtechniques, thus enabling cost effective manufacture to be achieved.Another object is to achieve improvements in highly collimated uniformlight emission.

SUMMARY OF THE INVENTION

According to the invention, there is provided an illuminator comprisingan array of light sources mounted in cavities in a substrate, and anelectrical drive circuit, wherein the substrate comprises anelectrically insulating body plated with conductors for the drivecircuit.

In one embodiment, the substrate body is of a circuit board material.

In another embodiment, the substrate body is of FR4 material.

In a further embodiment, the conductors extend into the cavities to alsoact as reflective coatings on the cavity walls.

In one embodiment, the conductors extend underneath the light sources.

In another embodiment, the light sources are bare semiconductor die.

In a further embodiment, the illuminator further comprises a thermallyconductive structure under the light sources.

In one embodiment, the thermally conducting structure comprises aplurality of layers bonded to a surface of the substrate body.

In another embodiment, the thermally conductive structure comprises aheat spreader in direct contact with a plating under a light source.

In a further embodiment, the heat spreader comprises a metal platingpatterned onto the substrate under each cavity.

In one embodiment, heat spreader comprises a plurality of metal coatingspatterned onto the substrate, one under the other.

In another embodiment, there is one heat spreader per light source.

In a further embodiment, the thermally conducting structure comprises aglobal thermally conducting layer underneath all of the cavities.

In one embodiment, said global layer comprises a resin embedded withthermally conductive particles.

In another embodiment, the particles are of diamond material.

In a further embodiment, the particles are of a ceramic material such asBoron Nitride.

In one embodiment, the thermally conductive structure further comprisesa heat sink bonded to the global layer.

According to another aspect there is provided a method of producing anilluminator comprising the step of:

-   -   providing a substrate body of insulating material,    -   completing a substrate by plating the body with an electrically        conductive plating;    -   forming an array of cavities in the substrate at a top side, the        cavities having a shape for desired light reflection; and    -   placing a light source in each cavity.

In one embodiment, the plating of the substrate is patterned after thecavity-forming step to both provide the drive circuit and opticallyreflective coatings on the walls of the cavities.

In another embodiment, the substrate is plated with metal on anunderside, and each cavity is formed through the full depth of thesubstrate body to expose the plating on the underside.

In a further embodiment, the cavities are formed by drilling.

In one embodiment, the invention comprises the further steps of applyinga thermally conductive structure to the underside of the substrate.

In another embodiment, the thermally conductive structure is applied tothe platings under the cavities and exposed substrate surfacestherebetween.

In a further embodiment, an additional metal layer is applied to theplatings before application of the thermally conductive structure.

In one embodiment, the thermally conductive structure comprises a layerof resin impregnated with thermally conductive particles.

In another embodiment, a heat sink is applied to said layer.

In a further embodiment, the heat sink and the resin layer are appliedwith use of adhesives and pressing.

DETAILED DESCRIPTION OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example onlywith reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic cross-sectional side view of a part of anilluminator circuit of the invention; and

FIG. 2 is a diagrammatic cross-sectional view of a part of analternative illuminator for an outdoor signage application.

DESCRIPTION OF THE EMBODIMENTS

The invention provides an illuminator comprising an array of bare dieLEDs with a close packing density, good thermal diffusion, and highoptical efficiency. The illuminator is manufactured using conventionalcircuit board manufacturing techniques and materials and so excellentmanufacturing efficiency can be achieved. One light unit 1 of theilluminator is shown in FIG. 1, the others being similar and beingmanufactured simultaneously.

An LED 2 is mounted in a cavity 3 having a round shape in plan, a flatcircular base 3(a) and a tapered side wall 3(b). The cavity 3 is formedin an FR4 substrate 7 and is coated with a reflective coating 4(a). Thereflective coating is also a conductor forming part of the drive circuitof the illuminator. The LED 2 is secured to the cavity base 3(a) byconductive adhesive 5. The reflective coating 4(a) extends over the Cuconductor plating 6(a) around the rim of the cavities 3.

The substrate 7 is an FR4 printed circuit board having top Cu plating6(a) and bottom Cu plating 6(b).

A high thermal conductivity prepreg layer 8 is bonded to the bottomsurfaces of the substrate 7 and platings 6(b) and 4(b). The substrate 7has small cavities or wells 10 in its bottom surface to accommodateexcess adhesive when the layer 8 is being bonded. Such wells may beavoided if the copper 6(b) pattern on the underside of the body 7 issuch as to provide the same effect. A heat sink 9 is bonded to the layer8 underneath the LEDs 2 by thermally conductive epoxy.

The process for producing the illuminator is as follows. These stepssimultaneously manufacture the full illuminator with all of the lightsources in an array.

-   -   (a) The substrate 7 comprising the FR4 body and the platings        6(a) and 6(b) is provided. In a pre-processing step these        coatings 6(a) and 6(b) are deposited in a desired pattern using        conventional circuit board manufacturing techniques. The        substrate 7 is drilled with tapered drill bits conforming to the        shape of the cavity 3. A lubrication is used to provide a smooth        finish. Drilling is continued through the full depth of the FR4        body of the substrate 7 until the drill bit exposes the lower Cu        plating 6(b).    -   (b) The base 3(a) and the tapered side wall 3(b) of the cavity        3, and the underneath coating 4(b) are then coated using        conventional PCB conductor patterning steps including plating        through holes in PCBs. In this embodiment, the plating sequence        is 20-40 microns of matt Cu, a thin layer of bright Cu, 2-5        microns of bright nickel, and a sub-micron layer of gold. Other        alternative reflective coatings will be known by those skilled        in the art.    -   (c) The plating 6(a) and 6(b) is then etched to form the desired        final circuit patterns for connectivity of the LEDs 2 and other        components. This final pattern outside of the cavities may        alternatively be made in the initial substrate preparation stage        when the platings 6(a) and 6(b) are being deposited on the FR4        board.    -   (d) The layer 8 and the heat sink 9 are then applied using        adhesives and a pressing operation performed with insertion of a        support in the cavities 3. The heat sink 9 is an anodising        layer. A support may alternatively be avoided if plating        thickness and geometry provide sufficient strength for pressing.        An important aspect of the layer 8 is that it has high thermal        conductivity for dissipation of heat to the heat sink 9. The        layer 8 comprises a prepreg resin with embedded diamond        particles at a concentration in excess of 30% to achieve good        thermal conductivity. The thermal conductivity is of the order        of 120-140 W/m° C. In other embodiments, ceramic particles such        as Boron Nitride may be used instead.    -   (e) Finally, the LEDs 3 are placed on the bases 3(a) with        thermally conductive adhesive deposits 5.

It will be appreciated that the invention achieves use of standard PCBprocessing techniques and materials to achieve excellent reflectivity,heat dissipation, and connectivity requirements of an LED array in asimple and compact manner. The configuration of the cavities 3 can bechosen in a versatile manner to suit the particular application, a drillbit with a different profile being used. The bonding and pressingoperations achieve excellent physical strength together with the desiredheat dissipation to the heat sink 9. The adhesive 5, the reflective(metal) coating 4(a), the base Cu copper 6(b), the coating 4(b), and thelayer 8 provide excellent thermal conductivity to the heat sink 9.Indeed, it is envisaged that the heat sink may be dispensed with forsome configurations and/or light sources. Heat dissipation isparticularly assisted by virtue of the fact that the layers 6(b) and4(b) act together as a heat spreader for fast dissipation from the LEDs.

Referring to FIG. 2 an illuminator 30 has deep cavities 31 so that thereis a large degree of internal reflection of light emitted by the LEDs32. The side walls of the cavities 31 are deep enough to act like ashield from external light such as strong sunshine. This avoids the needfor external mechanical shields, thus reducing costs and complexity.This illuminator is particularly suitable for applications such asroadside signs, where there is considerable ambient light. Theconstruction under the LEDs 32 is similar to that shown in FIG. 1, thesedetails being omitted from FIG. 2.

It will be appreciated that the invention achieves a high light sourcedensity, excellent optical output and efficiency, and excellent heatdissipation despite the fact that conventional PCB processing techniquesand materials are used. Heretofore, the approach has been to try toobtain the above combination of desired optical properties whileaccepting that expensive manufacturing techniques and materials arenecessary.

The invention is not limited to the embodiments described but may bevaried in construction and detail. For example the PCB may bemulti-layer, incorporating plating internally, and possibly alsointernally incorporating a layer such as the layer 8. Also, thesubstrate body may be of any suitable material other than FR4. Animportant criterion is that the material is of a type which can bedrilled to expose a smooth surface which can accept a reflector layer.There may be vacuum deposition of parts. The layers underneath the lightsources may be of a different materials having a high thermalconductivity. Also, the cavity may be filled or partially filled withphosphorescent material, such as for producing white light from a bluesource. It is also envisaged that a lens may be mounted within the fieldof emission of the LED, possibly using an over-moulding process. Thecavity may have a different shape such as frusto-pyrmadial, parabolic,elliptical, or hyperbolic for example. The shape is in general chosenfor optimum reflectivity. High thermal conductivity particles other thandiamond or ceramics may be used in a resin layer. Also, the heatspreaders may compromise a material similar to that of the global layer8.

1. An illuminator comprising an array of a plurality of light sourcesmounted in a plurality of cavities in a substrate, and an electricaldrive circuit, wherein the substrate comprises an electricallyinsulating body plated with plural conductors for the drive circuit. 2.An illuminator as claimed in claim 1, wherein the electricallyinsulating body is of a circuit board material.
 3. An illuminator asclaimed in claim 2, wherein the electrically insulating body is of FR4material.
 4. An illuminator as in claim 1, wherein the plural conductorsextend into the plural cavities, whereby said plural conductors act asreflective coatings on the plural cavity walls.
 5. An illuminator asclaimed in claim 4, wherein the plural conductors extend underneath thelight sources.
 6. An illuminator as claimed in claim 1, wherein theplural light sources comprise bare semiconductor dies.
 7. An illuminatorcomprising an array of a plurality of light sources mounted in aplurality of cavities in a substrate, an electrical drive circuit,wherein the substrate comprises an electrically insulating body platedwith plural conductors for the drive circuit, and a thermally conductivestructure under the plural light sources.
 8. An illuminator as claimedin claim 7, wherein the thermally conductive structure comprises aplurality of layers bonded to a surface of the substrate body.
 9. Anilluminator as claimed in claim 8, wherein the thermally conductivestructure comprises at least one heat spreader in direct contact with aplating under a light source.
 10. An illuminator as claimed in claim 9,wherein the heat spreader comprises a metal plating patterned onto thesubstrate under each cavity.
 11. An illuminator as claimed in claim 9,wherein heat spreader comprises a plurality of metal coatings patternedonto the substrate, one under the other.
 12. An illuminator as claimedin claim 9, wherein the at least one heat spreader comprises one heatspreader per light source.
 13. An illuminator as claimed in claim 7,wherein the thermally conducting structure comprises a global thermallyconducting layer underneath all of the cavities.
 14. An illuminator asclaimed in claim 13, wherein said global layer comprises a resinembedded with thermally conductive particles.
 15. An illuminator asclaimed in claim 14, wherein the particles are of diamond material. 16.An illuminator as claimed in claim 14, wherein the particles are of aceramic material.
 17. An illuminator as claimed in claim 16 wherein theceramic material is Boron Nitride.
 18. An illuminator as claimed inclaim 13, wherein the thermally conductive structure further comprises aheat sink bonded to the globally conducting layer.
 19. An illuminator asclaimed in claim 7, wherein the electrically insulating body is of acircuit board material.
 20. An illuminator as in claim 19, wherein theelectrically insulating body is of FR4 material.
 21. An illuminator asin claim 7, wherein the plural conductors extend into the pluralcavities, whereby said plural conductors act as reflective coatings onthe plural cavity walls.
 22. An illuminator as claimed in claim 21,wherein the plural conductors extend underneath the light sources. 23.An illuminator as claimed in claim 7, wherein the plural light sourcescomprise bare semiconductor dies.
 24. A method of producing anilluminator comprising the step of: providing a substrate body ofinsulating material, completing a substrate by plating the body with anelectrically conductive plating; forming an array of cavities in thesubstrate at a top side, the cavities having a shape for desired lightreflection; and placing a light source in each cavity.
 25. A method asclaimed in claim 24, wherein the plating of the substrate is patternedafter the cavity-forming step to both provide the drive circuit andoptically reflective coatings on the walls of the cavities.
 26. A methodas claimed in claim 24, wherein the substrate is plated with metal on anunderside, and each cavity is formed through the full depth of thesubstrate body to expose the plating on the underside.
 27. A method asclaimed in Claim 24, wherein the cavities are formed by drilling.
 28. Amethod as claimed in claim 25, comprising the further steps of applyinga thermally conductive structure to the underside of the substrate. 29.A method as claimed claim 28, wherein the thermally conductive structureis applied to the platings under the cavities and exposed substratesurfaces therebetween.
 30. A method as claimed in claim 29, wherein anadditional metal layer is applied to the platings before application ofthe thermally conductive structure.
 31. A method as claimed in claim 29,wherein the thermally conductive structure comprises a layer of resinimpregnated with thermally conductive particles.
 32. A method as claimedin claim 31, wherein a heat sink is applied to said layer.
 33. A methodas claimed claim 32, wherein the heat sink and the resin layer areapplied with use of adhesives and pressing.